Reducing the environmental impact of curable compositions, such as that associated with emissions into the air of volatile organic compounds during the application of curable coating compositions, has been an area of ongoing investigation and development in recent years. Accordingly, interest in high solids liquid coating compositions has been increasing, due in part to their comparatively lower volatile organic content (VOC), which significantly reduces air emissions during the application process.
Lower VOC coating compositions are particularly desirable in the automotive original equipment manufacture (OEM) market, due to the relatively large volume of coatings that are used. However, in addition to the requirement of lower VOC levels, automotive manufacturers have very strict performance requirements for the coatings that are used. For example, automotive OEM top coats are typically required to have a combination of good exterior durability and excellent gloss and appearance.
Thermosetting coatings containing hydroxyl functional polymers have been used extensively as automotive OEM primers and top coats. Such coating compositions typically comprise a crosslinking agent having at least two functional groups that are reactive with hydroxyl groups, and a hydroxyl functional polymer. The hydroxyl functional polymers used in such coating compositions are typically prepared by standard, i.e., non-living, radical polymerization methods, which provide little control over molecular weight, molecular weight distribution and polymer chain structure.
The physical properties, e.g., viscosity, of a given polymer can be directly related to its molecular weight. Higher molecular weights are typically associated with, for example, higher Tg values and viscosities. The physical properties of a polymer having a broad molecular weight distribution, e.g., having a polydispersity index (PDI) in excess of 2.5, can be characterized as an average of the individual physical properties of and indeterminate interactions between the various polymeric species that comprise it. As such, the physical properties of polymers having broad molecular weight distributions can be variable and hard to control.
The polymer chain structure, or architecture, of a polymer can be described as the sequence of monomer residues along the polymer back bone or chain. A hydroxyl functional copolymer prepared by standard radical polymerization techniques will contain a mixture of polymer molecules having varying individual hydroxyl equivalent weights and polymer chain structures. In such a copolymer, the hydroxyl functional groups are located randomly along the polymer chain. Moreover, the number of functional groups is not divided equally among the polymer molecules, such that some polymer molecules may actually be free of hydroxyl functionality. In a thermosetting composition, the formation of a three-dimensional crosslinked network is dependent on the functional equivalent weight as well as the architecture of the individual polymer molecules that comprise it. Polymer molecules having little or no reactive functionality (or having functional groups that are unlikely to participate in crosslinking reactions due to their locations along the polymer chain) will contribute little or nothing to the formation of the three-dimensional crosslinked network, resulting in decreased crosslink density and less than optimum physical properties of the finally formed polymerizate, e.g., a cured or thermoset coating.
The continued development of new and improved thermosetting compositions having lower VOC levels and a combination of favorable performance properties is desirable. In particular, it would be desirable to develop thermosetting compositions that comprise hydroxyl functional copolymers having well-defined molecular weights and polymer chain structure, and narrow molecular weight distributions, e.g., PDI values less than 2.5.
International patent publication WO 97/18247 and U.S. Pat. Nos. 5,763,548 and 5,789,487 describe a radical polymerization process referred to as atom transfer radical polymerization (ATRP). The ATRP process is described as a living radical polymerization that results in the formation of polymers having predictable molecular weight and molecular weight distribution. The ATRP process of these publications is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). These patent publications also describe polymers prepared by ATRP, which are useful in a wide variety of applications, for example, with paints and coatings.
It would be desirable to develop thermosetting compositions that comprise hydroxyl functional copolymers prepared using atom transfer radical polymerization, thus having well-defined molecular weights and polymer chain structure, and narrow molecular weight distributions. Such compositions would have lower VOC levels due to lower viscosities, and a combination of favorable performance properties, particularly in coatings applications.